In a gas and steam turbine system the heat contained in the expanded operating medium or heating gas from the gas turbine is used for the generation of steam for the steam turbine. The heat is transferred in an exhaust heat steam generator connected downstream from the gas turbine, in which a number of heating surfaces are usually arranged for water pre warming, for evaporation of the water and for steam superheating. The heating surfaces are connected into the water-steam circulation of the steam turbine. The water-steam circulation usually comprises a number of pressure stages, for example three, with each pressure stage able to have a continuous evaporating heating surface.
There are a number of alternative design concepts to be considered for the steam generator connected downstream from the gas turbine on the heating gas side as a waste heat steam generator, namely a throughflow steam generator design or a recirculating steam generator design. With a throughflow steam generator the heating of the steam generator pipes provided as evaporation pipes leads to an evaporation of the flow medium in the steam-generating pipes in a single pass. By contrast, with a natural or forced circulation steam generator, the water introduced into the circulation is only partly evaporated when it passes through the evaporation pipes. The water not evaporated in this case is fed back again, once it has been separated from the steam generated, to the same evaporation pipes for a further evaporation.
By contrast with a natural or forced circulation steam generator a throughflow steam generator is not subject to any pressure limiting so that fresh steam pressures far in excess of the critical pressure of water (PKri≈221 bar)—where only small differences in density between liquid-like and steam-like medium—are possible. A high fresh steam pressure promotes a high degree of thermal efficiency and thereby low CO2 emissions of a fossil fuel-fired power station. In addition a throughflow steam generator has a simpler construction compared to a recirculating steam generator and can thus be manufactured at particularly low cost. The use of a steam generator designed in accordance with the flowthrough principle as a waste heat steam generator of a gas and steam turbine system is thus particularly useful for achieving a high overall efficiency of the gas and steam turbine system with a simple construction.
A waste heat steam generator constructed in a horizontal manner has particular advantages as regards the effort involved in manufacturing it, but also as regards the required maintenance work, the heating medium or heating gas, that is the waste gas from the gas turbine, being fed in an approximately horizontal direction of flow through the steam generator. In a horizontal-design throughflow steam generator the steam-generation pipes of a heating surface can however be subjected to widely different heating, depending on their positioning. Especially with steam generation pipes connected on the output side to a common collector, a different heating of individual steam generation pipes can result in a merging of steam flows with sharply differing steam parameters and thereby to undesired losses of efficiency, especially to a comparatively reduced effectiveness of the heating surface involved and thereby to reduced steam generation. A different heating of adjacent steam-generating pipes can also, especially in the inlet area of collectors, lead to damage to the steam-generating pipes or to the collector. The use of a horizontal design of throughflow steam generator, which is desirable per se, as a waste heat steam generator for a gas turbine, can thus present significant problems as regards a sufficiently stabilized flow feed.
A steam generator is known from EP 0 944 801 B1 which is suitable for an arrangement as a horizontal design and also features the stated advantages of a throughflow steam generator. To this end the known steam generator is designed as regards its continuous evaporating heating surface so that a steam-generatng pipe which is heated more in comparison with a further steam-generating pipe of the same continuous evaporating heating surface has a higher throughflow of the flow medium compared to the further steam-generating pipe. The continuous evaporating heating surface of the known steam-generator thus exhibits a self-stabilizing behavior in the type of flow characteristics of a natural recirculating continuous evaporating heating surface (natural recirculating characteristic) on occurrence of different heating of individual steam-generating pipes which without requiring any outside influence, leads to a balancing out of the outlet-side temperatures even at differently heated, flow-medium side steam-generating pipes connected in parallel. However the known steam generator is comparatively expensive to construct, especially as regards the water and/or heat-side distribution of the flow medium.